1. Field of the Invention
The invention relates to a light source device for a data projector, a liquid crystal projector, or a DLP projector.
2. Description of Related Art
Recently, a liquid crystal projector and a DLP projector have increasingly come into use which incorporate digital-light-processing technology (DLP(trademark) from Texas Instruments). In these projectors, a so-called video monitor of the front projection type is used in which monitor light from a projector is projected onto a screen which is located at a distance from the above described projector. They are used by increasing the radiance of the light source and by improving the degree of light utilization of the liquid crystals and DLP, even in bright spaces. Furthermore, recently a so-called video monitor of the rear projection type or rear projection system as a monitor such as a television for home use in which a light source device and a screen are made integral with one another, has become more and more popular.
FIGS. 1(a) and 1(b) schematically show a conventional video monitor of the rear projection type 100 in a cross section. In the video monitor of the rear projection type 100, a light source device 200, optical parts 6 of the illumination system, a rear reflector 3 and a screen 4 are provided in a cage-like body 5.
The light source for a video monitor of the rear projection type is a short arc discharge lamp such as a metal halide lamp, a super-high pressure mercury lamp, or the like. The trend is for the thickness of a video monitor from the rear projection type to become smaller based on the market demands for saving space.
There are many cases in which according to the continuing increase in radiance, the above described super-high pressure mercury lamp is used as the light source. Since in a super-high pressure mercury lamp, a higher mercury operating pressure can be achieved by vertical luminous operation than by horizontal luminous operation, vertical luminous operation is desirable. In addition, in a super-high pressure mercury lamp by vertical luminous operation, a heat-resistant construction of the arc tube can be enabled with relative ease, with consideration of the lamp properties, such as the increase of radiance, prolongation of the service life and the like, thereby making vertical luminous operation especially desirable.
In FIG. 1(a), the screen 4 is vertical. To form a quadratic projection surface on the screen 4 in which the top and the bottom are the same size, the projection distance between the light source and the top side of the screen 4, and the projection distance between the light source and the bottom of the screen 4, must be the same size. When these distances are different, a trapezoidal projection surface is formed on the screen 4. When the rear reflector 3 is tilted to a great degree to eliminate the above described defect, the depth of the video monitor of the rear projection type 100 becomes large as shown in FIG. 1(a).
Therefore, as shown in FIG. 1(b), an approach was found to tilting the illumination light by means of the optical parts 6 of the illumination system while maintaining vertical luminous operation of the discharge lamp 1. But for this reason, there had to be more optical parts 6 of the illumination system in the cage-like body 5 of the product which reduces the degree of freedom of the arrangement of the rear reflector 3. As a result, the disadvantage was that it is difficult to reduce the thickness of the video monitor of the rear projection type.
FIG. 2 shows a video monitor of the rear projection type 100 in which the operating position of the light source device 200 is tilted with respect to vertical, making it possible to reduce the number of optical parts 6 of the illumination system which contain a direction limiting mirror, a prism and the like, as compared to the case of vertical luminous operation of the light source device 200 as shown in FIG. 1(b). Thus, the arrangement of the rear reflector 3 is simplified and the thickness of the video monitor of the rear projection type 100 is more easily reduced.
By reducing the number of optical parts of the illumination system 6 the cost can furthermore be reduced.
For the above described reasons, with respect to the implementation of a thickness-reducing construction of the product, there is a great market potential for luminous operation of the discharge lamp 1 of the light source device 200 tilted with respect to vertical luminous operation in the manner shown in FIG. 2.
However, the present inventors have found a limitation in the above described case of tilted luminous operation of the light source device 200, in that there is a disadvantage of the continued existence of a coil arc as compared to vertical luminous operation. Usually, for the cathode of the discharge lamp 1 under consideration, the cathode is wound with a component which is called the coil which serves as the starting point for the arc. After starting the discharge lamp, the coil loses its primary function and the hot spot of the arc is shifted from the coil to the tip of the upholding part of the cathode according to the increase of operating pressure in the arc tube. The above described disadvantageous phenomenon of continued existence of the coil arc is when the above described shift of the hot spot of the arc from the coil, to the tip of the upholding part of the cathode, does not take place. Instead, in coil arc, the discharge remains stable, not between the electrode tips where actually the arc gap should be, but the discharge remains stable between the coil and the anode. The coil is, however, a component which is necessary for facilitate starting the discharge lamp.
In a discharge lamp which is operated using a direct current, the cathode and the anode are heated along the lengthwise axis, sealed and thus secured. But since the anode has a greater weight than the cathode, a slight eccentricity forms on the tip of the upholding part of the anode with respect to the lengthwise axis of the lamp. Between this eccentric tip of the upholding part of the anode, and the coil with which the cathode is wound, the arc shifts during luminous operation. There are cases in which the generated coil arc continues to exist depending on the position of the eccentric tip of the upholding part of the anode.
When a coil arc forms, the focussing point of the concave reflector and the middle position of the arc diverge from one another so that sufficient utilization of the light intensity as a light source becomes more difficult. Furthermore, as a result of the continued existence of the coil arc, the tungsten which comprises the coil vaporizes. The vaporized material is deposited on the inside wall of the arc tube which consists of quartz glass, and the transmittance of the emission part is reduced. This results in the disadvantages that the light intensity on the projection surface is attenuated. In addition, the emission part is deformed as a result of the local temperature increase and thus, the lamp properties are adversely affected.
In view of the above, the primary object of the present invention is to devise a light source device in which, in a discharge lamp used for the light source device of a video monitor of the rear projection type, uninterrupted formation of a coil arc is prevented thereby prolonging the service life of the lamp. Another object of the present invention is to reduce the number of optical parts of the video monitor of the rear projection type, and to reduce its production costs.
In accordance with one embodiment of the described invention, in a light source device used for an irradiation light source of a video monitor of the rear projection type in which the emitted video light is reflected by a rear reflector located tilted in the horizontal direction and is emitted onto an essentially vertical screen, the advantages are attained in that the above described light source device comprises a concave reflector, and a discharge lamp with an arc tube with an emission part and hermetically sealed portions adjoining it laterally. The discharge lamp is operated using a direct current and is mounted with the hermetically sealed portion in a center opening of the above described concave reflector. In the arc tube, there are a pair of electrodes, specifically a cathode and an anode, opposite one another, so that the optical axis of the concave reflector is positioned tilted from the vertical direction essentially in the direction of the rear reflector, preferably toward its middle, so that the discharge lamp of the direct current luminous operation type is arranged with its anode at the top, and that the tip of this anode is positioned with respect to the lengthwise axis of the lamp, towards the side on which the above described light source device is tilted.
The primary object is furthermore achieved in another embodiment by the tilt angle of the above described light source device to the vertical axial direction preferably being greater than or equal to 5 degrees, and less than or equal to 50 degrees.
The primary object is moreover achieved in yet another embodiment by the lamp wattage of the above described discharge lamp of the direct current luminous operation type being preferably greater than or equal to 100 W. In still another embodiment, the above described arc tube contains greater than or equal to 0.16 mg/mm3 mercury, and a rare gas or a rare gas and halogen. In yet another embodiment, the primary object is also achieved by having the wall load of the discharge lamp greater than or equal to 0.8 W/mm2.
These and other objects and advantages of the present invention will become more apparent from the following detailed description of the invention when viewed in conjunction with the accompanying drawings.